This invention relates to functional adjustment of a thick-film circuit containing a thick-film thermistor. More specifically this invention involves a thick-film thermistor-resistor network having a unique configuration which permits rapid functional adjustment of the thermistor as well as the resistor, and to a method of functionally adjusting this network.
Thick-film circuits are conventionally made by silk screening resistive and conductive compositions onto a ceramic substrate in a selected pattern. The substrate is then fired to fix the properties of the compositions and bond them to the substrate surface. Thick-film printed resistors are normally produced with resistance values that vary within rather wide tolerances. When higher accuracy is necessary, it is customary to functionally adjust the more significant thick-film printed resistors, and sometimes all of the resistors, to more precise resistance value.
A thick-film printed resistor is functionally adjusted by cutting a transverse groove partially across the resistor, deeper than resistor film thickness, to reduce resistor effective width. The resistance value of the resistor is substantially continuously monitored while the groove is being cut. When the resistance increases to a predetermined value, cutting of the groove is stopped. The groove can be cut by a narrow jet of air carrying a fine abrasive. This air abrade technique, in principle, should be useful in trimming a thick-film printed thermistor to an accurate preselected value. However, the abrasion of the thermistor material heats the thermistor, changing its resistance value while it is being cut. Concurrently, the jet of air carrying the abrasive has a cooling effect, which also changes the resistance value. Neither effect is consistent or known by applicants to be controllable. Trimming a thermistor by this technique is possible by successively cutting, checking, recutting, rechecking, etc. However, this is impractical for high volume commercial production applications. Too much time would be lost waiting for the temperature of the thermistor to equilibrate after each cut and recut. Thick-film printed resistors can also be trimmed using a laser beam to cut the transverse groove, instead of the air abrade technique. However, the laser beam heats the thermistor material as the thermistor is being trimmed. Consequently, laser trimming in the usual manner also is not practical for high volume commercial production applications. Hence, where a precise thermistor value was needed in a thick-film circuit made in high volume, it was not normally practical to include it as a thick-film component.
We have now found a unique thick-film circuit pattern for a thick-film printed thermistor that can be rapidly functionally adjusted by either an air borne abrasive or a laser beam. It can now be trimmed in the same manner and at the same speed as a resistor. Accordingly, we have discovered a new way of trimming a thick-film printed thermistor that is practical for high volume commercial production applications.